Thursday, 13 July 2017

Larsen C just broke off from Antarctica

A
chunk of floating ice that weighs more than a trillion metric tons
broke away from the Antarctic Peninsula, producing one of the largest
icebergs ever recorded and providing a glimpse of how the Antarctic
ice sheet might ultimately start to fall apart.

A
crack more than 120 miles long had developed over several years in a
floating ice shelf called Larsen C, and scientists who have been
monitoring it confirmed on Wednesday that the huge iceberg had
finally broken free.

There
is no scientific consensus over whether global warming is to blame.
But the landscape of the Antarctic Peninsula has been fundamentally
changed, according to Project Midas, a research team from Swansea
University and Aberystwyth University in Britain that had been
monitoring the rift since 2014.

“The
remaining shelf will be at its smallest ever known size,” said
Adrian Luckman, a lead researcher for Project Midas. “This is a big
change. Maps will need to be redrawn.”

Larsen
C, like two smaller ice shelves that collapsed before it, was holding
back relatively little land ice, and it is not expected to contribute
much to the rise of the sea. But in other parts of Antarctica,
similar shelves are holding back enormous amounts of ice, and
scientists fear that their future collapse could dump enough ice into
the ocean to raise the sea level by many feet. How fast this could
happen is unclear.

In
the late 20th century, the Antarctic Peninsula, which juts out from
the main body of Antarctica and points toward South America, was one
of the fastest-warming places in the world. That warming had slowed
or perhaps reversed slightly in the 21st century, but scientists
believe the ice is still catching up to the higher temperatures.

Some
climate scientists believe the warming in the region was at least in
part a consequence of human-caused climate change, while others have
disputed that, seeing a large role for natural variability — and
noting that icebergs have been breaking away from ice shelves for
many millions of years. But the two camps agree that the breakup of
ice shelves in the peninsula region may be a preview of what is in
store for the main part of Antarctica as the world continues heating
up as a result of human activity.

“While
it might not be caused by global warming, it’s at least a natural
laboratory to study how breakups will occur at other ice shelves to
improve the theoretical basis for our projections of future sea level
rise,” said Thomas P. Wagner, who leads NASA’s efforts to study
the polar regions.

The
time-lapse image below shows the rift gradually widening from late
2014 to January of this year.

In
frigid regions, ice shelves form as the long rivers of ice called
glaciers flow from land into the sea. The result is a bit like a clog
in a drain pipe, slowing the flow of the glaciers feeding them. When
an ice shelf collapses, the glaciers behind it can accelerate, as
though the drain pipe had suddenly cleared.

At
the remaining part of Larsen C, the edge is now much closer to a line
that scientists call the compressive arch, which is critical for
structural support. If the front retreats past that line, the
northernmost part of the shelf could collapse within months.

“At
that point in time, the glaciers will react,” said Eric Rignot, a
climate scientist at the University of California, Irvine, who has
done extensive research on polar ice. “If the ice shelf breaks
apart, it will remove a buttressing force on the glaciers that flow
into it. The glaciers will feel less resistance to flow, effectively
removing a cork in front of them.”

Scientists
also fear that two crucial anchor points will be lost.

According
to Dr. Rignot, the stability of the whole ice shelf is threatened, as
the shelf front thins.

“You
have these two anchors on the side of Larsen C that play a critical
role in holding the ice shelf where it is,” he said. “If the
shelf is getting thinner, it will be more breakable, and it will lose
contact with the ice rises.”

Ice
rises are islands that are overridden by the ice shelf, allowing them
to shoulder more of the weight of the shelf. Scientists have yet to
determine the extent of thinning around the Bawden and Gipps ice
rises, though Dr. Rignot noted that the Bawden ice rise was more
vulnerable.

“We’re
not even sure how it’s hanging on there,” he said. “But if you
take away Bawden, the whole shelf will feel it.”

The
Antarctic Peninsula may be
a canary in a coal mine.

The
collapse of the peninsula’s ice shelves can be interpreted as
fulfilling a prophecy made in 1978 by a renowned geologist named John
H. Mercer of Ohio State University. In a classic paper, Dr. Mercer
warned that the western part of Antarctica was so vulnerable to
human-induced climate warming as to pose a “threat of disaster”
from rising seas.

He
said that humanity would know the calamity had begun when ice shelves
started breaking up along the peninsula, with the breakups moving
progressively southward.

The
Larsen A ice shelf broke up over several years starting in 1995; the
Larsen B underwent a dramatic collapse in 2002; and now, scientists
fear, the calving of the giant iceberg could be the first stage in
the breakup of Larsen C.

“As
climate warming progresses farther south,” Dr. Rignot said, “it
will affect larger and larger ice shelves, holding back bigger and
bigger glaciers, so that their collapse will contribute more to
sea-level rise.”